Over the last several years, many fiber optic network providers have included 1550 nm links in their newer network buildouts, some of which have older fibers, creating networks in which 1550 nm source lasers transmit into conventional optical fibers optimized for 1310 nm transmission. One advantage of 1550 nm transmission is its superior attenuation performance. In conventional SiO.sub.2 -based single-mode optical fibers, for example, the attenuation of 1550 nm optical signals is about 0.2 dB/km, while the attenuation of 1310 nm signals in the same fiber is about 0.35 dB/km. The lower optical loss at 1550 nm allows for longer, unrepeatered link spacings.
Another advantage of 1550 nm transmission is the recent commercial availability of low cost, low noise rare earth ion doped optical fiber amplifiers. The development of these amplifiers has made it possible to reamplity and distribute signals without optical detection and regeneration, resulting in substantial cost savings. To date, the most widely used rare earth ion doped amplifiers make use of erbium doped fibers that operate in the range from about 1520-1565 nm. Because of these advantages, transmission at wavelengths of 1550 nm has become a practical and desirable option to traditional 1310 nm transmission.
Despite the advantages of 1550 nm operation, unacceptable levels of chromatic dispersion can occur when 1550 nm wavelengths are transmitted through conventional optical fibers. Dispersion is a linear effect present in an optical fiber whereby different frequencies of light travel with different group velocities through the fiber. Fiber optimized for 1310 nm wavelength transmission has a zero-dispersion wavelength at 1310 nm and dispersion of up to 17 ps/km/nm at 1550 nm. The higher dispersion at 1550 nm is a significant problem in longer links.
The effects of dispersion are particularly pronounced when present with any type of source chirp. The term "chirp" generally refers to the amount of frequency modulation or phase modulation of the optical signal induced by the intensity modulation process. In CATV networks, the combination of either type of chirp and fiber dispersion results in a significant dispersion penalty as the peaks of the AM signals are delayed relative to the valleys, resulting in unacceptable signal distortion. This distortion primarily manifests itself in a frequency-dependent contribution to system level composite second-order distortion ("CSO"). In a distributed feedback ("DFB") laser, applying the signal directly to the laser leads to a large amount of chirp. Accordingly, direct modulation of the 1550 nm source laser is generally not a viable option in most existing fiber optic links having fibers optimized for 1310 nm. Instead, relatively low chirp external modulation is typically used.
One commonly used external modulator is known as the Mach Zehnder modulator. When driven in the push-pull configuration, the Mach Zehnder produces an output signal exhibiting substantially zero chirp. Although beneficial for reducing the effects of dispersion in 1550 nm links, a drawback of the conventional Mach Zehnder modulator is that its output does not exhibit a high degree of linearity. In practice, second order distortion is eliminated by appropriately biasing the modulator. However, separate electrical predistorters are used to eliminate third order distortion. By using separate components for linearization, however, environmental stability, component matching and other considerations must be taken into account.
In response to the limitations in the linearity of the traditional Mach Zehnder, optically linearized modulators have been developed that include a pair of cascaded Mach Zehnder modulators. By appropriately varying the bias current and the amount of modulating voltage applied to each of the individual Mach Zehnders, both second and third order distortion components are canceled and a relatively high degree of linearity can be achieved, obviating the requirement of a separate predistorter. One drawback of these modulators for 1550 nm links is that they induce a substantial amount of modulator chirp, approximately 3.5 MHz/1% OMI at 500 MHz, where OMI refers to the optical modulation index. Although the magnitude of the modulator chirp is generally much smaller than that of laser chirp in a DFB laser, in conjunction with dispersion the modulator chirp caused by the cascaded Mach Zehnder detrimentally affects performance in any 1550 nm link having 1310 nm optimized fibers, and is unacceptable in longer, high performance 1550 nm links.
In view of the foregoing, those skilled in the art would prefer an optical modulator that exhibits the relatively high degree of linearity of the cascaded Mach Zehnder modulators, and also the substantially zero chirp exhibited by the traditional Mach Zehnder. Such a modulator would be particularly useful for enabling 1550 nm transmission over existing fibers having a zero-dispersion wavelength centered around 1310 nm.